It is common to use mechanical dot matrix printers for high speed applications. Generally, the dot matrix printers utilize a hammerspring with a tip at the end thereof to impact a ribbon. The ribbon impaction is then received as a printed dot on paper that is to be printed upon and is supported by a platen.
The series of dots printed on the paper provide letters, numbers, and other symbols on the paper. A very common use today of dot matrix printers is the printing of bar codes.
Bar codes are becoming prevalently used in a greater number than ever before. During the printing of such bar codes, it is common to use impact printing mechanisms consisting of a hammerspring to achieve the printed dots that conform to the bar code that is to be printed. This is particularly true with respect to dot matrix printers that are known as line printers.
Prior art hammerspring designs are generally of a configuration having a uniform thickness and width throughout the spring. This physical configuration is in the nature of a leaf spring.
Such leaf springs do not provide the capability of storing energy in an efficient and effective manner as the invention hereof. When storing the energy for release of the hammerspring, a higher energy storage provides for a better printing force, a faster cycle time and more importantly, significant life. The leaf spring hammerspring designs of the prior art did not achieve the cycle times of this invention in combination with the life and force of impact as is provided by the hammerspring of this invention.
There are various regions of concern in the action of a hammerspring for a dot matrix printer and in particular a line printer. These regions are in association with a series of hammerbank permanent magnets that hold the hammersprings in a retracted position under a magnetic force. When the magnetic force is released by electrical coils, the hammersprings are able to fire into a ribbon with the print tips or pins in order to effectuate a printing through the ribbon onto the paper.
A fundamental design constraint is the force that can be utilized through the poles of the permanent magnets in order to hold the hammersprings. This is critical with regard to the air gaps and the nature of the material being used for the hammerspring.
A criteria as to the aspects of retaining the hammersprings is such as to allow a maximum rate of firing to meet a specified number of lines per minute of the printer. The force requirement for retaining the hammersprings by overcoming their elastic nature must be in balance with the hammerspring material. The hammersprings must not only provide for suitable mechanical properties, but also magnetic properties and magnetic retention through mechanical design, shape, and metallurgical requirements.
The invention hereof provides for magnetic retention, through a design which has a sufficient cross section and mass to obtain a required magnetic force for retaining the hammerspring. On the other hand, in order to maintain rapid firing, it is necessary to minimize the mass of the hammerspring. In effect, it is necessary to produce maximum force with a hammerspring with minimum mass.
The inventors hereof have been able to achieve this by having a large cross sectional area in the magnetic field between the pole pins of the permanent magnets to support the flux therein. The magnetically conductive circuit is optimized to allow for a substantial amount of magnetic flux to flow while at the same time minimizing the mass of the hammerspring.
Another important concern is in the hammerspring spring region which provides the stored energy. The spring region should be such that it will provide substantially infinite life over the life of the printer. It must also have an excellent dynamic response and adequate frequency response.
The prior art leaf type hammersprings do not provide for these effects in the most efficacious manner. This invention overcomes the prior art deficiencies by allowing the width and the thickness of the hammersprings to be gradually reduced along the length of the hammerspring portion. This results in a uniform stress, along the active spring storage portion of the hammerspring rather than a peak of stress at the maximum bending moment position.
Finally, the area where the hammerspring is to be clamped has to be designed such that any one hammerspring is isolated from the other hammersprings. Any one hammerspring's behavior should not influence or be influenced by a neighboring hammerspring. Furthermore, once the hammersprings have been emplaced, they should not have to be reset and should have constant characteristics.
The inventors have done this by providing for uniform hammersprings on a fret. These frets are preestablished and can be moved from one location to the other on a hammerbank without re-calibrating them. In effect, the inventors have been able to provide for a uniform hammerspring action once the hammersprings have been manufactured and emplaced.
The invention hereof is a significant step over the art with respect to hammersprings in their configuration and operation. The net result is to provide for a hammerspring and printer with a hammerbank which significantly improves the operation and life over that of the prior art.